The present invention relates to a method of manufacturing a ceramic circuit board particularly provided with a circuit pattern with highly improved positioning performance and capable of achieving yielding of the circuit boards with high quality.
In the known art of this field, there have been widely utilized ceramic circuit boards, for power transistors, microwave transistors, module structures, hybrid integrated circuit (IC) boards and the like, each in which a metallic circuit plate formed from copper, for example, having a high heat conductivity and high electrical conductivity is bonded to at least one of surfaces of a ceramic insulating plate.
In the known art, such ceramic circuit boards have been manufactured by forming plural kinds of circuit pattern elements by punching out a metallic conductive plate such as copper plate with a punching press, arranging these circuit pattern elements in predetermined positions on the ceramic board base and then integrally bonding them by means of heat bonding, for example. In this manufacturing method, there has been adapted a method, as the bonding step, in which a metallic material and the ceramic board base are directly bonded by forming on a bonding surface an eutectic fused layer formed of the metallic material and oxygen, or in which active metal is diffused in both the ceramic board base and the circuit pattern elements to thereby integrate them with each other.
It is however considerably troublesome to fix the plural circuit pattern elements at their respective predetermined positions on the ceramic board base, resulting in increased working steps, thus being disadvantageous.
In order to obviate the above defect of the prior art, there has been also provided a circuit pattern forming method in which a circuit pattern base including a plurality of circuit pattern elements, which are preliminarily coupled with each other through bridging pieces, is prepared by means of press working. The thus prepared circuit pattern base and a backing copper plate are bonded to the front and back surfaces of the ceramic board base, respectively, and the respective bridging pieces are then cut off by a cutting tool such as rotary cutter, thus forming the circuit pattern having a predetermined shape.
In such conventional method, however, it is difficult to substantially completely remove the respective bridging pieces by using the rotary cutter and burrs or the like remaining unremoved are formed at the bridging piece removed portions. Such burrs may cause an abnormal discharge when the circuit board is utilized and the withstand voltage characteristics of the circuit board is hence largely lowered, thus adversely affecting the reliability of the whole circuit board structure or performance.
In the meantime, since the circuit pattern base in the conventional technique is prepared by the press punching working, large mechanical shock or impact is applied to the circuit pattern base, which results in increased deformation of the circuit pattern base. The deformation of the circuit pattern base further results in inadequate contact with the ceramic board base and, hence, the bonding defects between the ceramics board base and the circuit pattern base occurs with relatively high frequency. Particularly, in these days, since there has been generally adapted a method in which the eutectic fused layer formed of the metallic material and the oxygen is formed on the bonding surface to thereby directly bond the metallic material and the ceramic board base, the defects in bonding working rapidly increase, resulting in substantial reduction of the production of ceramic circuit board products, even in the slight deformation of the circuit pattern base.
Furthermore, in the method in which the circuit pattern base is prepared by the pressing means, since a die pressing apparatus is itself expensive and the punching performance of the pressing apparatus is generally limited to about 1 mm in the minimum width, it is difficult to form a complicated circuit pattern with high density and high performance. Further, increased working costs are incurred.